This invention relates generally to power factor corrected power converters that work in the transition or discontinuous mode. More particularly, the invention relates to reducing total harmonic distortion and input current phase variation in such power converters.
Transition mode power converters have the desirable property that the current through the inductor after every switching cycle returns back to zero, so as to allow zero-current and zero-voltage switching of the main switch. While in theory the current of a power converter operating in the transition mode with a constant on-time switching scheme is always proportional to and in phase with the input voltage, practical limitations deteriorate the performance which results in a total-harmonic-distortion (THD) value that is unacceptable in certain applications.
One reason for the deviation from theoretical perfect behavior is that many transition mode controllers clamp the maximum switching frequency of the converter to limit the switching losses. As a result, the converter is not able to maintain the transition mode switching conditions and starts to work in the discontinuous switching mode where the average input current using a constant on-time switching scheme is not proportional to the input voltage.
Some transition mode controllers allow very high switching frequencies to improve the THD of the input current. This improves the THD at the cost of reduced efficiency due to increased driving and switching losses. However, the THD of the input current is still negatively affected close to the polarity changes of the input voltage. As a result of the small input voltage, the stored energy in the inductor becomes very small and is not able to commutate the boost diode due to the parasitic capacitances of the switching node.
Another technique for improving THD and input current phase matching is implementing a current mode controller, which automatically increases the on-time of the main switch close to the zero-crossing of the input voltage to follow the current reference, storing sufficient energy in the inductor to commutate the diode. A shortcoming of this method is that the average input current over one switching cycle needs to be sensed, which increases the cost and complexity for the control and current sensing.